M1 is stronger
The M1 was built for Macs, while the A15 was built for phones, the M1 was designed for Macs and has so far appeared in products like the MacBook Air, Mac Mini and iMac. The new A15Bionic will ship with the iPhone13, iPhone13Pro, and iPad Mini6, making it essentially smaller and more portable than a laptop.
The M1 chip is more powerful
In terms of performance, the M1 chip is about 50% faster than the A15 Bionic and 70% more powerful than the A14 Bionic (which is the chip in the 4th generation iPad Air). While the A15 Bionic has a 6-core CPU and 5-core GPU, the M1 chip is equipped with an 8-core CPU and 7-core GPU, as well as 8GB of RAM in the lowest configuration.